Geologic History of the Lower St. Louis River

The geologic history of the lower St. Louis River can be traced through the rocks and sediments found in its riverbed and along its shoreline. The river flows over bedrock of the Canadian Shield, the ancient, stable core of the North American continent.

The present St. Louis River channel was shaped primarily by glaciers during the Pleistocene epoch. As glaciers advanced and retreated across the land, receding for the last time around 10,000 years ago, the slowly melting ice and flowing meltwater left behind complex patterns of sediment, including moraines, drumlins, and lake button clays. These glacial deposits, which form many of the surface features we see today, greatly influence the flow and habitat conditions of the river. 

The lower St. Louis River flows through high banks of red clay, silt and sand, which were deposited when a large lake, known as Glacial Lake Duluth, covered the area. Glacial Lake Duluth formed as meltwater was trapped in front of the ice of the Superior Lobe when it filled the basin to the northeast. The red clay that is so typical of the lower St. Louis River and the Nemadji River basin was deposited in the deep water of this glacial lake.

As the ice receded farther north, lower outlets were exposed in other areas, dropping the lake level significantly as the water drained away to the east. This allowed water to flow into the lake at the western end, cutting a deep channel-the ancestral St. Louis River-into the easily eroded red clay. Then, as the heavy rate of the ice was removed, the land began to rebound. Since the land to the north was the last to lose its covering of ice, it was the last to rebound. As the land rose, the water in Lake Superior shifted toward the western end of the lake and flooded the channel of the St. Louis River and its tributaries, forming the freshwater estuary that we see today.

A baymouth sand bar formed across the western end of the lake, separating the estuary from the open water of the lake and creating a sheltered harbor. Historically, there was only one break in the baymouth bar where both the St. Louis River and the Nemadji River flowed out into Lake Superior.